Due to their poor solar light reflectivity, conventional black pigments used on the surface of architectural materials and the like raise temperatures of those architectural materials.
Hence, there have been made researches on a pigment (heat-reflective pigment) which can selectively reflect so-called heat rays (infrared light) while maintaining black color by suppressing the reflectance of visible light.
The present applicant proposes, in Patent Document 1, that a spinel-type crystalline structure affects selective reflectivity of infrared light, and that a non-spinel type crystalline structure improves to better reflect infrared light selectively.
Patent Document 2 proposes a non-spinel type (hematite type) pigment comprising Cr2O3 and Fe2O3 as main components. This pigment has high degree of selective reflectivity in the mid-infrared ray (wavelength: 2500 nm).
See also Non-Patent Document 1, concerning the selective infrared light reflectivity (hereinafter sometimes referred to merely as “infrared reflectivity”) of conventional pigments. Non-Patent Document 1 provides a figure showing the characteristics of high-solar-reflectance paints (see FIG. 1 attached to this application), and this document which is relevant to the characteristics of high-solar-reflectance paints is referred to herein by reference.
The present inventor has announced excellent infrared reflectance performance of a black pigment comprising a (Cr,Fe)2O3 solid solution, more specifically a black pigment comprising Cr and Fe at a ratio (molar ratio) of (80:20) and forming a non-spinel structure.
The characteristics of the black pigment is shown as Comparative Example 2 in FIG. 2.
FIG. 1 shows the light reflectance characteristics of both high solar reflectance paints and generic paints in relation to the lightness. In general, the lightness degree of 30 or less exhibits a black color.